Plants are hosts to thousands of infectious diseases caused by a vast array of phytopathogenic fungi, bacteria, viruses, and nematodes. These pathogens are responsible for significant crop losses worldwide, resulting from both infection of growing plants and destruction of harvested crops. The most widely practiced methods of reducing the damage caused by such pathogens involve the use of various chemical agents. Unfortunately, many pathogens develop resistance to such chemicals, and some pathogens (especially viruses) are not susceptible to control by chemical means. In addition, many of the chemical agents used are broad-spectrum toxins, and may cause serious environmental damage, as well as toxicity in humans.
Plant breeding and, more recently, genetic engineering techniques have also been employed to combat plant pathogens. In certain instances, breeders and molecular biologists have successfully engineered resistance to certain pathogens. In the last few years, a number of plant R (resistance) genes have been isolated from plants. When introduced into otherwise susceptible crops, these R genes produce enhanced resistance to certain pathogens. For example, U.S. Pat. No. 5,571,706 describes the isolation of the tobacco N gene, which confers enhanced resistance to Tobacco Mosaic Virus. However, while conventional breeding and genetic engineering approaches reported to date can successfully enhance pathogen resistance, they typically address problems caused by just one pathogen, or a small number of closely related pathogens. As a result, while crops produced using these approaches may have enhanced protection against one pathogen, conventional chemical agents must still be used to control others.
It would be of great agricultural benefit to be able to produce plants having enhanced resistance to a broad spectrum of pathogens, including bacterial and fungal pathogens. It is to such plants that the present invention is directed.